“No Categories” WHY?

Today, without any reason, my categories disappear from the blog homepage.

In the back end all is well. The categories are there. I can manage them, add categories to posts etc…

But in homepage the function
<?php wp_list_categories('title_li='); ?>
returns “No Categories”.

The wierd thing is that the theme is customized. If I switch to the default theme it works!!!!

Any ideas?